Examples of electronic handwriting equipment include portable/personal assistants (PDAs), palmtop computers and mobile telecommunication terminals (mobile phones). They have in common that they make use of a stylus and a touch-sensitive display screen, a solution that offers improved usability and flexibility compared to conventional user interfaces with a keypad or keyboard.
In an apparatus that relies primarily on a stylus and a touch-sensitive display screen for user input, the the stylus plays two roles; sometimes it works like a normal pen for writing and sometimes like a mouse for controlling the user interface. How to detect which role the pen is taking is important for reasons of efficiency, usability as well as accuracy.
In this document, when the real physical stylus works as a normal pen is referred to as a “logical pen” mode. Conversely, when the stylus works like a normal mouse in a computerized user interface is referred to as a “logical mouse” mode.
A stylus-based user interface can receive three kinds of events from a physical stylus, as detected by the touch-sensitive display screen: pen down, pen move and pen up. Each event has two common properties: the happening position and the time of the event. These three kinds of events form event circles like “pen down->pen move(s)->pen up”. There should be one pen down at the beginning of the event circle and one pen up at the end of the circle. The number of pen moves, however, can be any from 0 to virtually infinite.
When the stylus works as a logical pen, a grahical pen trace of the pen event circle is presented on the display screen. The pen traces are removed from the display screen when they make up and have been recognized as a complete symbol.
When the stylus works as a logical mouse, no trace is presented on the screen. However, objects or control elements such as buttons, icons and menu options on the display screen can be selected and operated by the stylus.
How to distinguish whether an event circle belongs to a logical pen mode or a logical mouse mode has been solved in the prior art in two different ways.
A first solution, which is illustrated in FIG. 4, is based on spatial information. The basic rules are:    When the pen is operated in a special area, this event is recognized as an event of a logical pen.    Otherwise, it is recognized as a logical mouse event.
The special area is normally called a writing area and is of limited size compared to the entire available area of the touch-sensitive display screen.
U.S. Pat. No. 6,567,549, in FIG. 7 thereof, illustrates the user interface of a palmtop computer, where the display has different limited writing areas, referred to as character input boxes 710, 760, 750. An important drawback of this solution is that it reserves a part of the display screen as special writing area, which cannot be used for other purposes. This special area cannot be too small since it must be able to contain the handwriting of characters and symbols. Particularly in mobile phones and other similar miniaturized devices, this becomes a serious problem, since the display screens of mobile phones are usually very small and the display screens must be used also for other purposes than receiving handwriting, namely for presenting both information and objects such as control elements of the user interface.
A second solution, which is illustrated in FIG. 5, is based on temporal information. Basically, for the following cases, the stylus will be recognized as a logical pen:                When there are one or more pen traces on the screen (step 510).        When there is no pen trace on the screen, the event circle contains at least one pen move event (step 520), and the duration Te between the pen down event and the first pen move event is less than a threshold Ts (step 530).        
For other cases, the stylus is considered as a logical mouse. Those cases can be concluded as follows:                When there is no pen trace on the screen and the event circle does not contain a pen move event.        When there is no pen trace on the screen, the event circle contains at least one pen move event but the duration Te between the pen down event and the first pen move event is not less than a threshold Ts.        
Normally the threshold Ts is very small, for example 0.5 seconds.
An important drawback of this solution is that it is not particularly friendly to users. For example, a small wobble of hand can accidentally change the meaning of actions. Thus, the rate of wrong actions could be rather high, especially in moving environments, like in a vehicle. It is particularly difficult to discriminate an intended pen-down event (logical mouse) from a very short writing event (logical pen), since in a moving environment the stylus may slide slightly when applying a pen-down, thereby strongly resembling a short writing event.
Another problem with known electronic handwriting equipment is how to handle different symbol sets (or character sets). The accuracy of the handwriting recognition is generally better for restricted symbol sets with a smaller number of symbols than for symbol sets with a larger number of symbols. Thus, Latin letters are normally included in one symbol set (or in fact often two; upper case and lower case), Arabic numerals in another, Chinese characters in yet another, etc. The prior art generally suggests three different ways of selecting a desired symbol set for handwriting recognition:
1. Selecting a menu option in a graphical user interface.
2. Selecting/tapping a symbol set mode icon.
3. Writing a special symbol set control stroke that will restrict the interpretation of succeeding (or preceding) stylus strokes to a particular symbol set.
U.S. Pat. No. 6,567,549, which has been referred to above, is an example of solution 2. The leftmost character input box 710 is for inputting Japanese Kanji symbols, and the rightmost box 750 is for Arabic numerals. The center box 760 is a combined input box for inputting Japanese Hiragana characters, Japanese Katakana characters or Western Roman characters depending on the current input mode. The current input mode for the center box 760 is selected by the user by tapping the stylus on a corresponding Hiragana, Katakana or Roman mode box, these mode boxes being provided as graphical icons adjacently to the character input boxes.
All three of the above solutions have a common drawback in that they require an extra action from the user to select the desired symbol set. This is both inconvenient and will slow down the speed of handwriting input.